Field of the Invention (Technical Field)
The present invention relates to apparatus for manufacturing composite materials of wood and plastic, particularly sheets of relatively high density wood-plastic composition, and especially to a manufacturing method for composite sheets upon which selected films can readily be adhered, and to composite sheets so manufactured.
Background Art
It is known to manufacture composite materials from mixtures of ground or milled wood and finely ground plastics. A method and apparatus for manufacturing such a composition is disclosed in U.S. Pat. No. 7,914,275 entitled “Apparatus for Making High Density Wood and Plastic Composites,” the entire disclosure of which is incorporated herein by reference. It is generally desirable to manufacture composite material in sheets or panels suitable for use in the construction of buildings, signs, and the like. Other wood particle and thermoplastic particle compositions and panels manufactured there-from, and modes of fabrication therefore, also are taught in my U.S. Pat. Nos. 6,632,387 and 6,586,504, whose complete contents also are incorporated herein by this reference.
Current manufacturing techniques for thermoplastic-fiber composite materials focus on the extrusion process, whereby a molten cellulose/thermoplastic mix is forced under high pressure through a forming die. The product is shaped and formed corresponding to the die, and then allowed to cool. In some instances, the cooling of the extruded product is accomplished by wrapping the extruded product around one or more large, chilled, cylindrical mandrels. Such a cooling technique, however, flexes the product resulting in unacceptable fracturing of sheet or panel product having a thickness of more than about 0.5 inches. Alternatively or additionally, known fabrication processes require post-extrusion cooling mechanisms that are dozens of feet long, demanding large manufacturing facilities. Known extrusion-type production devices generally also are limited as to both the width and thickness of panels or sheets than can effectively be produced.
A further shortcoming of known systems and methods for manufacturing cellulose/plastic composites is the difficulty in fabricating a composite panel to which a polymeric film may be easily and securely attached with an ordinary know, and perhaps yet-to-be-developed, adhesive. Known extrusion processes for wood-plastic composite panel fabrication result in a cooled panel with a very smooth surface having thereon a thin layer of melted polymers. This layer or coating is an unavoidable consequence of the heated extrusion process for extruding a wood-plastic composite. The thin layer manifests a slick, “oily,” character that resists adhesion thereto of most commonly used adhesives, especially adhesives by which, for example, a thin plastic layer or film (e.g., 3M® Type IV Prismatic Reflective Sheeting, or 3M® Diamond Grade™ Types IX and XI Reflective Sheeting) be attached to the panel surface. It is desirable to secure to a composite panel surface another layer, such as a thin plastic film, when for example it is desired to apply a visual sign or other layer to the cooled panel surface. Extruded wood-plastic composites exhibit excellent durability, strength, and weathering characteristics for use as the foundation for road signs, but many preferred types of film appliqués for use as signs are difficult to apply to such composites.
In summary, therefore, there is an unmet need for a method of manufacturing a wood-plastic composite material in sheets that can readily be cut into construction panels of conventional size (e.g. four feet by eight feet), which material is of adequate density and durability for use as a construction material, but which such fabrication method results in a panel with a working surface to which conventional adhesives readily adhere. It is especially desirable that the working surface need no pre-treatment prior to the application of an adhered film or layer, such as a surface coating or sign. Against this background, the present system and process was developed.